Further, Merriam-Webster defines it thus: a planting of low plants (as ivy) that covers the ground in place of turf.
Take a look at the pictures from my garden and decide if you see a ground cover or not. It's simple: YES or NO. Answers (or maybe just my opinion) are at the end.
A) YES, wild ginger
B) YES, creeping plhox
C) YES, I guess, vinca. Really, I just want the last of it gone from my yard
D) YES, creeping jenny
E) YES, for me; maybe not for you - greek oregano (depends on where you have it)
F) YES, woodland phlox (at least for me)
G) NO, monarda (bee balm) really looks like a creeper in early spring
H) YES, mondo grass
I) YES, english ivy - ground, log, and fence cover in my yard
J) NO, dianthus - cheddar pinks
K) YES, ajuga
L) YES, sedum
M) YES, sedum
N) NO, ox-eye daisies
O) NO, red dianthus
P) NO, mulched leaves; they cover the ground alright, but they're not growing
Q) NO, of course not - this is turf grass that hasn't turned green yet. If you missed this one you didn't pay attention to the definitions.
As you can see, sometimes whether a plant is a ground cover or not depends on how, where, and how much in the garden. And, NO, NO, NO, mulch and snow are NOT ground covers, even though they may cover the ground.